To date, microstructured materials have been obtained by rapidly solidifying composite metal(s), alloy(s), compound(s), etc., and most of the obtained microstructured materials have particle sizes of several microns. In recent years, research has been actively conducted seeking to minimize the size of materials, i.e., from the micron to nanometer order. One of the primary characteristics of nanostructures, such as nanoparticles, is that a high percentage of atoms exist on the particle boundary (surface), for example, the ratio may reach 40% in 5 nm nanoparticles. Nanostructured materials may have chemical and physical characteristics that differ greatly from those of microstructured materials having the same chemical composition, and nanostructured materials often exhibit desirable characteristics.
However, it is currently difficult to obtain, for example, manganese oxides (MnOx) in a nanostructured form. Usually, the particle size of transition metal oxides prepared for commercial use is in the level of microns. The characteristics of micron-scale manganese oxides when used as an oxygen reduction catalyst have been reported. For example, Patent Document 1 discloses that manganese oxide materials of different oxidation states (valencies) exhibit different catalytic activities, i.e., Mn2O3 and MnOOH, which are trivalent manganese compounds, have higher oxygen reduction catalytic activity than Mn3O4 and Mn5O8, which have different valencies, and the oxygen reduction potential of Mn2O3 and MnOOH is observed around −0.3 V and −1.0 V, respectively.
As a method for manufacturing nanostructured manganese dioxide (MnO2), for example, a method is known wherein an aqueous potassium permanganate (KMnO4) solution is sprayed onto an aqueous sulphuric acid solution comprising manganese sulfate (MnSO4) dissolved therein to produce a synthetic reaction, a reaction product is separated out, and the reaction product is subjected to heat treatment to obtain manganese dioxide (MnO2) (Patent Document 1, page 42, FIG. 2).
An example of an oxygen reduction electrode using a manganese oxide is an air-zinc battery, wherein a mixture of micron-scale trimanganese tetraoxide (Mn3O4) and manganese dioxide (MnO2) powder is used as the oxygen reduction electrode (Patent Document 2, page 8, FIG. 2).
In addition to Patent Documents 1 and 2, Patent Documents 3 and 4, and non-Patent Documents 1 and 2 can be cited as reference material relevant to the present invention.                [Patent Document 1] Japanese national publication of the translated version of PCT application No. 2000-505040        [Patent Document 2] Japanese Unexamined Patent Publication No. 1998-302808        [Patent Document 3] Japanese Unexamined Patent Publication No. 2000-144387 (in particular, paragraph [0015])        [Patent Document 4] Japanese Unexamined Patent Publication No. 2003-306319        [Non-Patent Document 1] Journal of The Electrochemical Society, 149 (4) A504-A507 (2002)        [Non-Patent Document 2] Laser Engineering, Volume 28, Number 6, June 2000, pp. 348 to 353.        